This invention relates generally to halftoning for reproduction of color documents, and more particularly to a method of maintaining the angles of each separation at a preferred difference.
Full color in printed documents is the result of rendering, in combination, a limited set of marking colors over a small area, in densities selected to integrate to a desired color response. This rendering is accomplished in many printing devices by reproducing separations of the image, where each separation provides a single primary color of varying density. When combined together with other color separations, the result is a full color image.
In the digital reproduction of documents, a color separation is conveniently represented as a monochromatic bitmap, which may be described as an electronic image with discrete image signals (hereinafter, pixels) defined by position and density. The density of a color at any particular pixel position is represented as one of a number of possible states or levels. When more than two levels of density are used in the description of the image, the levels are often termed xe2x80x9cgrayxe2x80x9d, indicating that they range between a maximum level and minimum level, and without reference to their actual color. Most printing systems have the ability to output or render an image with a small number of levels, typically two, although more output levels are possible. Common input devices including document scanners, digital cameras and computer image generators, however, are capable of describing an image with a substantially larger number of gray levels e.g., 256 levels is common, although larger and smaller numbers of levels are also possible. An image initially described with a large set of levels typically must be describable using a smaller set of levels, in a manner that captures the intent of the user, in order to allow rendering of the image as a hardcopy output. In digital printing of color documents this is accomplished by reducing the relatively large number of input levels in a color separation to a smaller number of output levels. The multiple color separations are then combined together at printing, for example in an image-on-image printing system, to yield the final color print. Commonly, color documents are formed using cyan, magenta, yellow and black colorants although alternative or other combinations of colorants may also be used.
In printing color documents, obtaining the desired density of a particular color is accomplished by halftoning, where separation density variation is represented by marking greater or fewer numbers of ON pixels (binary) in a distinct area of a separation. In a halftoning method known as dithering, or screening, a value representing the density of each color separation pixel of an array of separation pixels within the area is compared to one of a set of preselected thresholds (i.e., stored as a dither matrix). The repetitive output pattern generated by this comparison is considered a halftone cell as taught, for example, in U.S. Pat. No. 4,149,194 to Holladay. The effect of such an arrangement is that for an area where the image density of the color separation lies between the maximum and minimum levels, some of the thresholds within the dither matrix will be exceeded while others will not. In the binary case, the separation pixels or cell elements for which the thresholds are exceeded might be printed as a maximum colorant value, while the remaining separation pixels are allowed to remain white, dependent on the actual physical quantity described by the data.
While widely accepted as a method for rendering color prints of digital images, dithering creates problems where the repeating pattern of the screen, when superposed over similar repeating patterns in the multiple color separations, can cause moirxc3xa9 or other artifacts.
Holladay et al., U.S. Pat. No. 5,394,252, teaches that there are 2-way moirxc3xa9 patterns between the color separations, but that the angles are often chosen to maximize the frequency of the moirxc3xa9s, resulting in the xe2x80x9crosettesxe2x80x9d noted in magnified color halftones. This is true of both analog (photographic) and digital systems. However, Holladay et al. (""252) is directed to the interaction of the three most perceptible colors, cyan, magenta and black, and to the elimination of three-color moirxc3xa9 amongst such colors using a non-periodic screen. Holladay et al. further notes that error diffusion, because it operates on a pixel-by-pixel basis is non-periodic, mitigating the problems of moirxc3xa9. Along these same lines, U.S. Pat. No. 5,740,334 to Lin et al. teaches that using a stochastically screened yellow separation will have a minimum effect on the image.
U.S. Pat. No. 4,149,194 to Holladay, assigned to the assignee of the present invention, discloses that the angle of the halftone screen can be changed to generate screen patterns which do not strongly beat visually against each other, thereby reducing the objectionable moirxc3xa9. Noted as particularly critical are the angles between the most prominent colors, cyan, magenta and black. A typical arrangement of rotated screen angles is 90xc2x0 (or 0xc2x0), 15xc2x0, 45xc2x0, and 75xc2x0 for cyan, yellow, magenta, and black (C,M,Y,K) respectively. However, even at these angles, moirxc3xa9 is a problem.
U.S. Pat. No. 5,892,891 to Dalal et al. and assigned to the assignee of the present invention, discloses that in a xe2x80x9chi-fixe2x80x9d color printing system, wherein colorants of hi-fi colors beyond the regular CMYK primary colorants are available, a main gamut obtainable with the CMYK colorants only is mutually exclusive with at least one extended gamut in which a hi-fi colorant is used and a complementary one of the CMY colorants is excluded. Because the main and extended gamuts are mutually exclusive, no more than four colorants are used in any part of the image, and no more than four halftone screens need be used to obtain any desired color. Thus, mutually exclusive gamut colorants may use the same separation screen angle. 
The references discussed herein and above are incorporated by reference for their teachings.
In accordance with the present invention, there is provided a method of halftoning four color images for reproduction in a color printer, wherein three preferred halftone screen angles are provided, and a first separation causing limited moirxc3xa9 is printed at a same screen angle as a second separation, when the density of said second separation is relatively low.
In accordance with another aspect of the present invention, the described halftoning method is used in conjunction with a color printer in which gray component replacement is used.
In accordance with still another aspect of the invention, prior to implementing the above-described screen angle share, a suitability determination is made, to determine whether the image is suitable for use of the technique. In addition, even if suitable, a blending check and operation is desirable, to avoid undesired artifacts in the use of this process.
In accordance with yet another aspect of the invention, in a color reproduction device, reproducing images in at least four color separations, each color separation printed with a screen at a selected screen angle, a method of sharing screen angles between distinct color separations including: for a received image, determining image segment suitability for separation screen angle sharing; for an image determined to be suitable, for a particular image component, determining separations that will share screen angles; and processing said image component with shared screen angles.
In accordance with still another aspect of the invention, there is provided a four color printer, printing superposed separations with cyan magenta, yellow and black separations, each separation including at least a portion thereof halftoned, a method of sharing screen angles between distinct color separations including from said cyan, magenta and black separations, determining for a selected area one of said separations with a minimum density; printing cyan, magenta and black separations with halftones at widely separated screen angles; and printing said yellow separation at a same screen angle at a same screen angle as said separation with said minimum density.
The invention takes advantage of the fact that all four colorants normally used in color reproduction are rarely present in the same localized image region. In such an arrangement, the separation having the least significant impact on moirxc3xa9 generation can share a screen angle used by a separation having a greater impact on moirxc3xa9. In practice, many localized image regions do not possess high levels of C, M, Y and K signals simultaneously, which provides an opportunity to use another color""s screen angle adaptively for the Y signal instead of the traditional 90xc2x0 screen. As an example, yellow separation data can be adaptively placed at any open separation screen angle. Thus, a preferred screen angle of 30 degrees between separations can be maintained throughout most of the image.